This invention relates to an optical cable with one or more fiber optics conductors that are located in a strand with a polypropylene envelope or in a polypropylene support element, and with an outer jacket.
The prior art describes various realizations of optical cables with one or more fiber optics conductors. Generally, the fiber optics conductor or conductors are arranged in strands or support elements that protect them from mechanical damage during the manufacturing and laying of the cable. Common applications include hollow strands or tubes made of plastic that enclose one or more fiber optics conductors and are generally stranded around a central element of the cable that has a high tensile strength. Such a cable with hollow strands, which can be made of polypropylene, for example, is described in DE 25 13 724 B2. Alternatively, the fiber optics conductors of an optical cable are frequently inserted individually or in the form of ribbons in slot-shaped recesses in the surface of a support element that run approximately in the longitudinal direction ("slotted core cable"). Slotted support elements are generally made of a plastic that is generally reinforced by high tensile strength inserts made of glass fibers or steel, for example. Externally, the optical cable is preferably enclosed by an outer jacket made of polymer material.
Conventional optical cables are generally flammable on account of the polymer materials used. Even if the outer jacket is made of a non-flammable material, an additional problem in the event of a fire is that the flames are propagated by the materials of the cable core. EP 0 484 744 A2 teaches a method to improve the behavior of such a cable in the event of a fire. Hollow strands are filled with an oil-based flame-retardant compound. Even in this case, however, the mechanically solid components of the cable core are flammable. To simplify the handling of the cable, it is also desirable in many cases that the cable core should not contain any liquid filler compounds.
An extremely important consideration is the behavior of cables in the event of a fire in enclosed spaces and tunnel systems, such as in airports, railway stations or subways. Significant hazards and damage are caused by smoke particles that interfere with visibility, and by toxic gases that interfere with the escape of persons which frequently result in their death. For these reasons, when such cables are used in enclosed spaces and tunnels, not only is there a requirement that the cables be flame-retardant, but the cables must also contain no halogens and must bum with little smoke. To improve the ability of persons to escape, a low smoke density is a distinct advantage, above all during the initial stages of the fire.